Breaking Change: Set id_tokens:aud via variables

What

• Set id_tokens:aud via variables instead of !reference.
• Caution: Backward Compatibility
  • This feature was introduced in 16.1. This CI template doesn't support GitLab before version 16.1.
  • This update continue to support .google-oidc:variables:WI_POOL_PROVIDER (Our requirement was that the user set this value). So most of users can continue to keep current CI configuration.
    • But this update no longer supports .google-oidc:variables:AUDIENCE (This was an optional value. We recommended that users do not set this value).

Why

• Close #3 (closed)
• Variables is more flexible than !reference. For example, users can set different audience in the same job easily.